Filch is a Medeivil Curse-Word used to Address The Act of Thievery,
the harshness is essentially = to the F-Word the implication of being called a Filcher was = to being called a Waste of space on This earth but obviously shorter and MUCH More Harsh...

Imagine if the F-Word Meant You Don't Deserve to Live when used to refer to someone

such as F*cker or Mother F**ker and That's Basically what we've Got here.